This invention relates to the thermoforming of plastic plates or dishes and more particularly to thermoformed plastic plates having a novel rim profile that provides enhanced strength and stiffness. The invention further relates to a dual-member mold for thermoforming the plate and particularly to a mold whose male member is adapted to receive interchangeable ring inserts carrying decorative patterns, whereby the rim of plates produced in the mold can be embossed with different patterns with minor change to the mold itself.
The art of shaping or molding thermoplastic materials in the presence of heat, known as thermoforming, has been practiced in the prior art to produce disposable articles for serving food. In the specific case of plates or dishes, the operation has comprised preheating a sheet of foamed thermoplastic material and impressing the preheated sheet between corresponding male and female mold members, forcing the sheet to assume the configuration of the mold cavity. These plastic plates, however, often have less strength than is desirable since the plastic foam material is not itself structurally strong. For example, foamed polystyrene, although desirably light weight and insulating in nature, does not always provide sufficient strength in a plate or dish, particularly at the rim of a plate, which because of its contours, often contains stress concentration points at which a break or bend is more likely to occur.
Additional strength has been provided to plates by use of relatively thick sheets of thermoplastic material in the plate forming process, providing denser or thicker plates, but this solution is undesirable economically. In another procedure, U.S. Pat. No. 3,684,633 discloses a foamed polystyrene dish which is laminated with a thin film of an oriented thermoplastic material, such as biaxially oriented heat-sealable polystyrene film. Although plates as described in this patent have much improved structural and other characteristics, the preparation of such plates requires the additional laminating step, which undesirably increases the cost.
To meet consumer desires, plastic plates and dishes are often embossed on the rims with a decorative pattern. This pattern is applied to the rim during the thermoforming step itself, the mold carrying the corresponding shape of the pattern on an appropriate surface thereof. Embossing or imprinting the plate rim, however, often creates additional stress concentration points, by virtue of the surface deformation of the material that takes place during embossing, and therefore affects the structural integrity of the rim, leading to decreased strength.
Additionally, a major cost in the molding of plastic plates having differing rim patterns is the need to have molds corresponding to each different pattern. Molds are quite expensive, and a plastic plate manufacturer must either expend considerable sums to keep several molds ready or to produce entirely new molds whenever a new rim pattern is desired. Although there have been attempts to overcome this problem by using inserts in molds for embossing design, generally these inserts have been for patterns covering an insubstantial portion of the surface of the molded article, and have not been used for thermoforming foamed sheets into plates or dishes having decorative rim patterns. U.S. Pat. No. 3,380,121 discloses a multi-member mold for blow molding thermoplastic containers which has a removable insert in each member to allow the exterior of the containers to be changed by using different inserts. In these molds, however, the inserts define most of the interior mold surface, and the molds themselves are not used in thermoforming sheets, the manner in which plastic plates are made, but rather in blow molding operations.